The present invention relates to multistage centrifugal turbomachines and to centrifugal impellers for multistage centrifugal turbomachines, particularly, but not exclusively, for oil and gas applications.
A centrifugal turbomachine is a rotary machine where mechanical energy is transferred between a working fluid and a rotary assembly including at least one centrifugal impeller. In oil and gas application, where the fluid is typically a gaseous fluid, centrifugal turbomachines include compressors and expanders. A compressor is a turbomachine which increases the pressure of a gaseous fluid through the use of mechanical energy. An expander is a turbomachine which uses the pressure of a working gaseous fluid to generate mechanical work on a shaft of the rotary assembly by means of the expansion of the fluid in the impeller(s).
In uncompressible fluid, e.g., water, centrifugal turbomachines include pumps and turbine, which transfer energy between the fluid and the impeller in a way analogous to compressors and expanders, respectively.
In general, in all cases, the working fluid exchanges energy with the centrifugal machine by flowing in the centrifugal impeller along a radial outward direction, oriented from an axis of rotation of the impeller to a peripheral circumferential edge of the impeller.
In particular, the centrifugal impeller of a compressor turbomachine transfers the mechanical energy supplied by a motor that drives the turbomachine to the working gaseous fluid being compressed by accelerating the fluid in the centrifugal impeller. The kinetic energy imparted by the impeller to the working fluid is transformed into pressure energy when the outward movement of the fluid is confined by a diffuser and the machine casing.
Centrifugal turbomachines are frequently referred to as single stage turbomachines when they are fitted with a single impeller, or as multistage centrifugal turbomachines when they are fitted with a plurality of impellers in series.
A prior art embodiment of a multistage centrifugal compressor 100 is illustrated in FIG. 1, in an overall section view.
The multistage centrifugal compressor 100 operates a process gas between an input pressure and an output pressure which is higher than the input pressure. The process gas may, for example, be any one of carbon dioxide, hydrogen sulfide, butane, methane, ethane, propane, liquefied natural gas, or a combination thereof.
Compressor 100 comprises a stator 102 within which is mounted a rotary assembly 103 including a shaft 104, which carries a plurality of identical impellers (three impellers 110, 111, 112 in the embodiment in FIG. 1) in series. The shaft 104 extends along an axis of rotation Y of compressor 100, having an axial span A, measured from the first impeller 110 to the last impeller 112.
Each impeller 110, 111, 112 has a typical closed design configuration including an impeller hub 113, which closely encircles the shaft 104, and a plurality of rotary blades 108 extending between a rear impeller disc 123 and a front shroud 119. The impeller disc 123 comprises a front side 124, which supports the plurality of rotary blades 108, and a rear side 125, which is opposite to front side 124. Each impeller 110, 111, 112 respectively comprises a low-pressure inlet side 110a, 111a, 112a defined by an impeller eye 115 on the front shroud 109 and a high-pressure outlet side 110b, 111b, 112b defined by a peripheral circumferential edge of the impeller 110, 111, 112.
The multistage compressor 100 is subdivided into a plurality of stages 107a,b,c (three stages in the embodiment in FIG. 1), each stage 107a,b,c including a respective impeller of the plurality of impellers 110, 111, 112. Between the first and second stage 107a,b the stator 102 includes a passage 105 for a process gas flowing from the outlet side 110b of the first impeller 110 to the inlet side 111a of the second impeller 111. The passage 105 comprises a diffuser 126 downstream the outlet side 110b, a return channel 128 upstream the inlet side 111a and a U-shaped bend 127 connecting the diffuser 126 and the return channel 128. A plurality of stator blades 115 are provided in the return channel 128 for guiding the process fluid toward the inlet side 111a of the second impeller 111. The process gas flowing in the diffuser 126 is directed along a first outward radial direction orthogonal to the axis of rotation Y while the gas flowing in the return channel 128 is directed along a second inward radial direction oriented toward the axis of rotation Y, the bend 127 providing a 180° degree deflection of the gas flow.
Analogously, a passage identical to passage 105 is provided in the stator 102 for the same process gas flowing from the outlet side 111b of the second impeller 111 to the inlet side 112a of the third impeller 112.
The passage 105 is provided in a diaphragm 118 extending in the stator 102 from one to the following impeller of the series of impellers 110, 111, 112. The diaphragm 118 comprises a first portion 138 extending axially, i.e., along an axial direction parallel to the axis of rotation Y, from the diffuser 126 and the rear side 125 of the impeller disc 123 to the return channel 128, and extending radially, i.e., along a radial direction orthogonal to the axis of rotation Y, between the shaft 102 and the bend 127. A seal 130 is provided in the gap 131 between the first portion 138 of the diaphragm 118 for preventing the process gas from leaking through the gap 131. The diaphragm 118 comprises a second portion 139 extending axially from the return channel 128 to the following stage of the plurality of stages 107a,b,c. An impeller eye seal 140 of the labyrinth type is provided between an impeller eye of the front shroud 119 of each centrifugal impeller 110, 111, 112 and the respective portion 139 of the diaphragm 118, in order to prevent the fluid from leaking in the space between each impeller 110, 111, 112 and the respective portion 139, from the outlet high-pressure side of the impeller to the inlet low-pressure side thereof.
It would be desirable to reduce as much as possible the axial span A, in order to reduce the overall sizes, weight and, as a consequence, cost of the turbomachine. In addition an axial span reduction would result in an improved rotordynamic behaviour, improving the stability of the rotary assembly which depends on the ratio between axial and radial sizes.